In the developing nervous system, neurons must extend axons to their correct targets to form an axon scaffold upon which functional neuronal connections are made. The migrating growth cone at the distal axon tip senses and responds to guidance information. The aim of these studies is to use C. elegans to understand the cytoskeletal signaling networks that link axon guidance signals to changes in growth cone morphology and outgrowth via the actin cytoskeleton. In this proposal, genetic, molecular, and in vivo time-lapse growth cone imaging approaches will be used to dissect the roles of cytoskeletal signaling pathways in growth cone morphology and outgrowth. Many pathways and molecules have been identified that affect axon pathfinding. These experiments move beyond the gene by gene approach to studying axon pathfinding and instead are designed to understand how molecules relate to one another in pathways and networks to control axon pathfinding and growth cone morphology. The first aim tests the idea that CDC-42 acts upstream of Rac GTPases in axon pathfinding. The second aim focuses on the control of Rac GTPases by the TIAM-1/Still life Rac GTP exchange factor (GEF) downstream of CDC-42. Aim 2 also is designed to test the role of the Cdc42 GEFs UIG-1/Clg and EXC-5/Fgd1 on regulation of CDC-42 in axon pathfinding. The third aim is to probe the role of the MIG-15 NIK kinase and RACK-1/Receptor for activated C kinase and their interaction with UNC- 115/abLIM downstream of CDC-42 in a Rac-independent pathway. The fourth aim integrates the first three and is to characterize the effects of these pathways on growth cone filopodia formation and morphology during outgrowth. The results of these experiments will significantly contribute to the goal of understanding the cytoskeletal signaling networks involved in growth cone morphology and will begin to address the cellular roles of these distinct pathways in axon development.